Support saving browser metrics to disk and reading them during next run.

base/metrics/persistent_histogram_allocator.cc

Index: base/metrics/persistent_histogram_allocator.cc
diff --git a/base/metrics/persistent_histogram_allocator.cc b/base/metrics/persistent_histogram_allocator.cc
index 1d5c0ef0e1c5d90175f8456469b67502f34e1d28..36dcb20eef9d6ab3fb8d43241623d32afa434515 100644
--- a/base/metrics/persistent_histogram_allocator.cc
+++ b/base/metrics/persistent_histogram_allocator.cc
@@ -6,6 +6,7 @@
 
 #include <memory>
 
+#include "base/files/important_file_writer.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
@@ -563,6 +564,32 @@ GlobalHistogramAllocator::ReleaseForTesting() {
   return WrapUnique(histogram_allocator);
 };
 
+void GlobalHistogramAllocator::SetPersistentLocation(FilePath location) {
+  persistent_location_ = std::move(location);
+}
+
+bool GlobalHistogramAllocator::WritePersistentLocation() {
+#if defined(OS_NACL)
+  // NACL doesn't support file operations, including ImportantFileWriter.
+  NOTREACHED();
+  return false;
+#else
+  // Stop if no destination is set, perhaps because it has not been enabled.
+  if (persistent_location_.empty())
+    return false;

[Ilya Sherman, 2016/04/19 00:57:46]

Hmm.  Could this be a DCHECK instead?  That is, are there legitimate expected
use cases where the destination would not be set?

[bcwhite, 2016/04/19 16:33:37]

Right now, it's mostly unset because the experiment is only at 10%.  It could
also be unset if for some reason the process starts to exit (crashes?) before
higher layers start and set the destination.

+
+  StringPiece contents(static_cast<const char*>(data()), used());
+  if (!ImportantFileWriter::WriteFileAtomically(persistent_location_,
+                                                contents)) {
+    LOG(ERROR) << "Could not write persistent histograms to file: "
+               << persistent_location_.value();
+    return false;
+  }
+
+  return true;
+#endif
+}
+
 GlobalHistogramAllocator::GlobalHistogramAllocator(
     std::unique_ptr<PersistentMemoryAllocator> memory)
     : PersistentHistogramAllocator(std::move(memory)),